The prior art described two systems that are used for opening laser fuses. In one system, the laser energy is applied, to the fuse to be opened, by a linear-motor-driven positioning system. In this case, the wafer bears on a chuck that can be moved in one dimension by a linear motor. A laser scanning apparatus can be moved over the wafer in a second dimension, orthogonal to the first dimension, by means of a linear-motor-driven optical system. The combination of both linear motors covers the entire area of the wafer. In the other system, the laser beam is positioned on the wafer by a biaxial galvanometer system. In addition, the wafer moves in large steps under the galvanoelements of the system, since these can cover only a small rectangular region on the wafer.
The fundamental procedure when opening the laser fuses is the same in both systems and comprises a multistage process that is described below.
First, a so-called wafer alignment is carried out, in the course of which the positioning of the wafer is ascertained with the aid of an optically scanned markings on the wafer, for example. The wafer is brought into correspondence according to the control program controlling the system.
Next, the actual beam positioning system is moved to the first processing chip or process field on the wafer, where chip or field focusing is effected in order that the laser can apply its maximum energy on the surface of the chip or the field. Since the wafer alignment permitted only a coarse positioning of the wafer, a chip or field alignment is now carried out by the so-called alignment marks on the chips or fields. This is done by carrying out a multiple linear movement of the positioning systems in order to enable the alignment marks to be scanned. In other words, to guide the alignment marks through under the optical recognition systems so that the latter can utilize the occurrence of an alignment mark or the recognition of a pattern of alignment marks to perform a very exact determination of the wafer orientation.
After this fine orientation, it is now possible to process the laser fuses on the chip or the field by driving one of the linear motors or one of the galvanoelements, respectively. After reaching the expected fuse location of the next fuse to be processed, the laser is activated and severs the fuse. This is carried out successively for all fuses of a chip or field, so that crossing and repeating routes of the system can occur. After the processing of one chip or field has been concluded, the positioning system is moved such that the next chip or field comes into the coarse processing region and a new focusing and new chip or field alignment has to be carried out. This sequence is continued until all chips or fields on a wafer have been correspondingly processed.
The chips on a wafer are accommodated such that they are closely adjoined to one another two-dimensionally and in rectangular fashion. The alignment marks are generally positioned at locations where they are meaningful for the positioning of the chips or fields, and they do not disturb the actual circuit structures either. By contrast, the laser fuses are positioned where this results from the circuit layer construction. This results in, given identical chips on a wafer, the alignment marks running in mutually parallel rows and columns if a two-dimensional grid is imagined on the wafer. The fuses also lie in such a grid in lines and columns. However, these lines and columns for the alignment marks are generally not the same as the laser fuses. Therefore, after the alignment marks have been scanned, a chip or field has to be processed without the alignment mark recognition apparatus passing an alignment mark again. It is typically necessary, prior to the processing of the next chip or field, to specially search anew for the latter's alignment marks since a reorientation of the system may become necessary from chip to chip, in order to ensure the required accuracy.
Depending on the product, between a few hundred and a million laser fuses have to be opened per wafer. These laser fuses are arranged in a manner distributed over the wafer. Moreover, other laser fuses have to be opened on each wafer in order to comply with the respective individual conditions or defects of the individual wafers. The beam positioning process is thus unique for each wafer and, as a result, in addition to finding the alignment marks complex and very time-consuming, none of the commercially available systems permits parallel processing of a plurality of chips since the majority of positioning systems required for this would otherwise interfere with one another.
Consequently, the process described above is time-consuming and separates between focusing, alignment and processing process. Both focusing and alignment processes are carried out by linear movements.